Electromagnetic torque developed by the motor is _______ compared to shaft torque.

Question

Examoogle · Accepted Answer

Option D: more

Explanation & Concepts:
The electromagnetic torque (also known as developed torque) of a motor is always greater than the shaft torque (also known as output torque). This occurs because the electromagnetic torque must overcome both the internal rotational losses of the motor and the external load torque requirement.

Working Principle: In an electric motor, the conversion of energy follows the sequence: Electrical Power Input $\rightarrow$ Electromagnetic Power Developed $\rightarrow$ Mechanical Power Output. The electromagnetic torque ($T_e$) includes the torque required to overcome friction, windage, and core losses (collectively rotational losses), whereas the shaft torque ($T_{sh}$) is the remaining useful torque available at the shaft after these losses are subtracted.

Key Formulas: $T_{e} = T_{sh} + T_{rotational}$ — Total developed torque includes losses; $T_{sh} = \frac{P_{out}}{\omega_m}$ — Shaft torque formula

Key Points: Electromagnetic torque is generated within the armature or rotor.; Rotational losses include mechanical friction, windage, and stray load losses.; Shaft torque is the net output torque measured at the load end.; At no-load, $T_{sh} = 0$, but $T_e$ is non-zero to overcome internal friction.

Advantages: Distinguishes between internal performance and useful mechanical work.; Helps in calculating motor efficiency accurately.

Applications: Motor efficiency testing; Drive system design; Industrial automation

Additional Info: The rotational losses are generally assumed constant in many standard machine analysis problems unless otherwise specified.. Option A is incorrect because the motor does not gain torque beyond what is generated internally.. Option B is incorrect as it ignores the presence of inevitable mechanical/magnetic losses.

Answer: D is correct — The electromagnetic torque is greater than the shaft torque because it must account for both the output load and the internal rotational losses.

Exam Tip: In any machine, remember the sequence of power flow: Input $\rightarrow$ Air-gap Power $\rightarrow$ Mechanical Power (Developed) $\rightarrow$ Output. Efficiency always involves losses at each stage.

Examoogle · Answer

Option A: less

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Option B: same

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Option C: depends on motor design

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